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Calculate the pH of 0.105 M CCl_{3}COOH (trichloroacetic acid, pK_{a} = 0.52).
Write the reaction, write the mass action expression and find K_{a}, set up a table of concentrations, plug the equilibrium concentrations into the mass action expression, and then solve.










Initial 




Change 




Equilibrium 



[HA]_{init}/K_{a} = 0.105/0.30 = 0.35 < 100 so no approximation
Using the quadratic equation gives
x = –0.38 or x = 0.082; only the positive value makes sense so
x = [H_{3}O^{+}] = 0.082 M
pH = –log[H_{3}O^{+}] = –log[0.082] = 1.09